This invention relates generally to driver circuits and more particularly to PIN diode driver circuits.
As is known in the art, a radar system typically includes an antenna, a transmitter and a receiver. A radar system transmitter generally includes an oscillator such as a magnetron oscillator, which may be turned on and off to generate a series of pulses. The pulses are fed to the antenna and emitted from the radar system.
It is also known in the art that RF receivers used in such radar systems generally include circuit components, such as low noise amplifiers (LNAs) which are susceptible to burn out from high power RF signals. High RF input signal levels provided by leakage from the radar's transmitter or from hostile jammers, for example, pose a threat to those circuit components of the RF receiver which are susceptible to burn out as a result of high incident power levels. Therefore, RF receivers having circuits susceptible to burn out need to be protected from input signals having high power levels.
One approach used to protect an RF receiver in a radar system is an active PIN diode limiter circuit. An active PIN diode limiter circuit includes a driver circuit and at least one, preferably two or more, PIN diodes. The PIN diodes are mounted in shunt between a transmission line and a reference potential. The active limiter is placed between the input signal source, such as the antenna for example, and the circuit which needs protection such as the RF receiver.
The active limiter operates in two basic modes. While operating in its normal, non-limiting mode, the driver circuit places the PIN diodes in a reverse biased conductive state and thus the active limiter provides a relatively low insertion loss characteristic to input signals fed thereto. To operate the active limiter in a limiting mode the driver circuit places the PIN diodes in a forward biased conductive state and thus the active limiter provides a very high insertion loss characteristic to input signals fed thereto.
The radar system operates in two modes. The first mode of operation is the receive mode. In the receive mode, the active limiter in its low loss state couples RF signals from the antenna to the receiver with minimum attenuation of the RF signals. However, if the input signal to the limiter exceeds a certain predetermined threshold, the driver circuit provides a forward bias voltage to the PIN diodes to place the active limiter into its limiting mode. In its limiting mode the active limiter provides a relatively high insertion loss characteristic to signals being fed from the antenna to the receiver.
The second mode of operation is the transmit mode. When the radar system is in the transmit mode, the transmitter provides a signal to the antenna and the signal is emitted from the radar system.
In those radar systems which use a common antenna to both transmit and receive signals, a component such as a duplexer, a circulator or the like provides isolated signal paths which couple the transmitter and the receiver to the common antenna. Components such as duplexers, circulators or the like, however, have a finite isolation characteristic. Thus, during transmit, portions of the transmit signal may leak back to the receiver due to the relatively poor isolation characteristics of the components which provide the receive path and transmit path to the common antenna.
To protect the receiver from the so-called leakage signals which are provided during the transmit mode, the active limiter must be in its limiting mode during the time when the transmitter provides a transmit signal to the antenna. In this instance, the driver circuit provides a forward bias voltage to the PIN diodes of the active limiter and thus places the active limiter in its limiting mode.
A so-called pulsed radar system determines the range to a target by measuring the time it takes a transmitted signal pulse having a predetermined pulse width to travel to the target and return. Once the transmitted signal pulse is emitted by the radar through the antenna, a sufficient time interval must elapse before the radar system transmits another signal pulse. The time interval allows any reflected signals to return to the radar and be detected by the radar's RF receiver before the radar system transmits the next signal pulse. The rate at which the pulses are provided by the radar system's transmitter is called the pulse repetition frequency (PRF).
As mentioned above, the active limiter should provide a high insertion loss characteristic during the time when the transmitter is providing a signal pulse to the antenna. However, the active limiter should provide a low insertion loss characteristic when the transmitter is not providing a signal to the antenna. Thus, the driver circuit should switch the PIN diodes to their high insertion loss state for a length of time substantially corresponding to the pulse width of the pulse signal.
Similarly, the driver circuit should switch the PIN diodes to their low insertion loss state when the transmitter is not providing a pulse signal. Thus, signal pulses reflected from the targets and received by the antenna are coupled to the receiver via the active limiter having a relatively low insertion loss characteristic.
For the active limiter to provide a relatively low insertion loss characteristic when reverse biased, the PIN diodes of the active limiter should have a low resistance. To handle high RF power levels, the diode should have a relatively large cross-sectional area. A PIN diode capable of handling RF signals having high power levels is typically provided having a relatively large cross-sectional area including a thick intrinsic (I) region having a minority carrier lifetime typically of about 450 nanoseconds. Such diodes are generally referred to as high power PIN diodes.
The thick I region provides a relatively insignificant time delay to the injection of charge carriers into the diode to forward bias the diode. The thick I region, however, provides a significant delay in the removal of the charge carriers when the PIN diode switches from the forward bias conductive state to the reverse bias non-conducting state. Thus, it is relatively difficult to rapidly switch high power PIN diodes between their conducting and non-conducting states.
In a radar system having a narrow pulse width and a high PRF, the driver circuit should rapidly switch the PIN diodes between their conducting and non-conducting states. To accomplish this, the driver circuit should alternatively provide a forward bias current and a reverse bias voltage to the PIN diode and thus rapidly move the charge into and out of the PIN diode.
Conventional PIN diode driver circuits are provided by bi-polar junction transistors which are physically small and which in their conductive state provide a relatively high resistance path between the voltage source and the diode. To bias high power PIN diodes, the transistors of conventional driver circuits switch between a positive DC voltage source typically of about 5 volts and a large negative voltage source typically of about 40 volts. Thus, the bi-polar junction transistors provide the forward and reverse bias signals to the PIN diode.
Several problems exist with this approach. First, the relatively high resistance path of the transistor coupled to the PIN diode limits the amount of current which the driver circuit may provide to a load such as a PIN diode for example. This driver circuit induced current limitation may be overcome by providing the driver circuit with a large bias voltage. By using a large negative bias voltage source the driver circuit can provide a large current, generally referred to as a reverse spike current, to the PIN diode. Typically, in the case of a high power PIN diode coupled to the driver, DC voltage supplies in the range of -40 volts are used to provide a large electric field and consequently a large reverse spike current to the PIN diode. Airborne radar systems, however, such as missile radars, are often limited to negative DC voltage supplies typically of about -25V.
A second problem arises when the minority carrier lifetime of the PIN diode is greater than the desired switching speed. The speed at which the diode switches is directly proportional to how rapidly the charge is removed from the diode. The charge will be removed from the diode due to the formation of electron-hole pairs provided in a process referred to as recombination. The minority carrier lifetime of the diode refers to the time it takes for substantially all of the electron-hole pairs to be formed in the diode. A reverse bias voltage provided to the PIN diode will promote the removal of I-region charge thereby increasing the speed at which the diode may switch between its conducting and non-conducting states. Thus when the minority carrier lifetime of the PIN diode is greater than the desired switching speed, the driver circuit should provide a large reverse bias voltage to the PIN diode in a small amount of time.
A third problem arises when the driver circuit should continuously switch a PIN diode between its conducting and non-conducting states. In a radar system having a narrow pulse width and a high PRF, for example, charge is moved into and out of the PIN diode repetitively through the relatively high resistance path of the transistors. Thus, the junction temperature of the driver circuit transistor coupled to the PIN diode rises significantly thereby compromising the reliability of the transistor and therefore compromising the reliability of the driver circuit.